The packaging of integrated circuit (IC) chips is one of the most important steps in the manufacturing process, contributing significantly to the overall cost, performance and reliability of the packaged chip. As semiconductor devices reach higher levels of integration, packaging technologies, such as chip bonding, have become critical. Packaging of the IC chip accounts for a considerable portion of the cost of producing the device and failure of the package leads to costly yield reduction.
As semiconductor device sizes have decreased, the density of devices on a chip has increased, along with the size of the chip, thereby making chip bonding more challenging. One of the major problems leading to package failure as chip sizes increase is the increasingly difficult problem of thermal coefficient of expansion (TCE) mismatches between materials leading to stress buildup and consequent failure. For example, in flip-chip technology chip bonding is accomplished by means of solder bumps formed on under bump metallization (UBM) layers overlying an IC chip bonding pad where, frequently, improper wetting (bonding) between the solder and UBM layers may lead to a bond not sufficiently strong to withstand such stresses.
In many cases it is necessary to repackage the chip after a package failure, requiring costly detachment of the chip from the package and repeating the chip bonding process in a new package. Some chip bonding technologies use a solder bump attached to a contact pad (the bonding pad) on the chip to make an electrical (and somewhat structural) connection from the chip devices to the package substrate. For example, C4 (Controlled-Collapse Chip Connection) is a means of connecting semiconductor chips to substrates in electronic packages. C4 is a flip-chip technology in which the interconnections are small solder balls (bumps) on the chip bonding pads. Since the solder balls form an area array (a “ball grid array” (BGA)), C4 technology can achieve a very high-density scheme for chip interconnections. The flip-chip method has the advantage of achieving a very high density of interconnection to the device with a very low parasitic inductance.
Solder bumps may be formed by, for example, vapor deposition of solder material over layers of under bump metallization (UBM) layers formed on the bonding pad. In another method, the layers of solder material may deposited by electro-deposition onto a seed layer material deposited over UBM layers formed on the bonding pad. In yet another method, solder bumps may be formed by a solder-paste screen-printing method using a mask (stencil) to guide the placement of the solder-paste. Typically, after deposition of the solder materials, for example, in layers or as a homogeneous mixture, the solder bump (ball) is formed after removing a photoresist mask defining the solder material location by heating the solder material to a melting point (a “reflow” process) such that a solder ball is formed with the aid of surface tension. Alternatively, a solder bump may be formed within a permanent mask made of photoresist or some other organic resinous material defining the solder bump area over the bonding pad. Because of the importance of the solder bumps/balls in such flip-chip techniques, improvements in processes used to form the solder balls on the IC chips are continuously being pursued.